1. Field of the Invention
This invention relates in general to motorcycle transmissions and in particular to motorcycle transmissions used on Harley-Davidson.RTM. Big Twin motorcycles, which have transmission cases that are characterized by being separate from the engine case and having concentric input and output members disposed on the same side of the transmission.
2. Description of the Prior Art
Different manufacturers of motorcycles, and different models from the same manufacturer, use different types and configurations of transmissions. The particular type of transmission used depends, amongst other things, on the history of the particular motorcycle, its intended uses, and it physical characteristics. Harley-Davidson.RTM. Big Twin motorcycles have a rather unique transmission in that the transmission case is truly separate from the engine case.
A brief description of the Harley-Davidson.RTM. Big Twin five speed transmission will help in understanding the present invention.
Referring first to FIG. 1A, the major components of a prior art Big Twin five speed transmission are shown in a partially exploded view. Many of the smaller components, which are well known to those skilled in the art, are not shown for clarity. A Big Twin five speed transmission case 10 is shown. The particular transmission case depicted as 10 is for Softail.RTM. models from 1989 to at least 1998; however, the invention is equally applicable to transmission cases for all other Softail.RTM. vintages and the other Big Twins (e.g. Softail.RTM., Dyna.RTM., FXR, and FLT), due to the large similarity. The transmission cases for the various Big Twins vary only in a few external dimensions that relate mainly to the attachment of the transmission case to, and fitment of the transmission case around, the particular frame and surrounding components of the particular Big Twin. The inner cavity and components of the transmissions for the various Big Twins are otherwise almost identical.
Transmission case 10 has a housing 12 with a central portion 14, left side wall 16, and a removable right side wall (or trapdoor) 18 defining a cavity 20. Recess 22 is located in left side wall 16 for receiving the left mainshaft case bearing 42, and recess 28 is located adjacent recess 22 for receiving the left countershaft case bearing 44. Recess 30 is located in trapdoor 18 for receiving the right mainshaft trapdoor bearing 46, and recess 36 is located adjacent recess 30 for receiving the right countershaft trapdoor bearing 48.
Referring still to FIG. 1A, five speed gear set 60 comprises mainshaft (or driveshaft) 62, countershaft 64, mainshaft 5th gear (also referred to as driveshaft drive gear or driveshaft output gear) 66, mainshaft 2nd gear 68, mainshaft 3rd gear 70, mainshaft 1st gear 72, mainshaft 4th gear 74, countershaft 5th gear 76, countershaft 2nd gear 78, countershaft 3rd gear 80, countershaft 1st gear 82, and countershaft 4th gear 84. Some of the components of five speed gear set 60 are not shown or are hidden from view; for example, split case bearings, retaining rings, thrust washers, and spacers.
To install five speed gear set 60 into transmission case 10, mainshaft 62 and countershaft 64 are first secured to trapdoor 18 by means of hex nuts 52. Right mainshaft trapdoor bearing 46 supports the right end of mainshaft 62. Right countershaft trapdoor bearing 48 supports the right end of countershaft 64. The gears and related hardware (split case bearings, retaining rings, thrust washers, and spacers) are then positioned about the mainshaft 62 and countershaft 64. Then, trapdoor 18, with five speed gear set 60 secured thereto, is fastened to the housing (by conventional fasteners which are not shown).
When five speed gear set 60 is installed into transmission case 10, the outer bearing surface 86 of mainshaft 5th gear 66 is secured by the left mainshaft case bearing 42. Mainshaft 5th gear bearings 50 (not shown in FIG. 1A, but well know in the industry) keep mainshaft 62 concentric with mainshaft 5th gear 66. The outer splined portion 88 of the mainshaft 5th gear 66 and the left end of mainshaft 62 protrude outside of transmission case 10. The left end of countershaft 64 is received by left countershaft case bearing 44.
Referring still to FIG. 1A, shifting assembly 94 is shown. Shifting assembly 94 comprises shifter cam 96 supported by left and right shift cam support towers 110 and 112. The shift grooves 98, 100, 102 are shown, but, for convenience, have not been accurately depicted because they are well known. Left and right shift cam support towers 110 and 112 are connected to the top of housing 12. Shifter forks 114, 116, and 118 are supported by shift fork shaft 120, which is in turn supported by shift fork shaft support holes 122 and 124.
In operation, when the shifter shaft (not shown) rotates in response to the rider operating the shift lever (not shown), the shifter pawl (not shown) acts against the shift pins 104 causing shifter cam 96 to rotate. The interaction of shift cam follower 108 and detents 106 biases the shifter cam 96 to predetermined angular positions of the shifter cam 96. The rotation of shifter cam 96 causes one or more of shifter forks 114, 116, 118 to move axially along shift fork shaft 120, in turn causing the appropriate gear members to move axially along either mainshaft 62 or countershaft 64, causing a change in the gear ratio.
Referring now to FIG. 1B, a schematic horizontal cross section of empty transmission case 10 is shown to better illustrate the location of, and respective distances between, bearings 42, 44, 46, and 48. Left countershaft case bearing 44 is pressed into recess (or hole) 28. Left mainshaft case bearing 42 is held in position within recess 22 by bearing shoulder 24 and by a snap ring (not shown) held by snap ring groove 26. Right countershaft trapdoor bearing 48 is held in position within recess 36 by bearing shoulder 38 and by a snap ring (not shown) held by snap ring groove 40. Right mainshaft trapdoor bearing 46 is held in position within recess 30 by bearing shoulder 32 and by a snap ring (not shown) held by snap ring groove 34.
The distance from the outside of bearing shoulder 24 (i.e. the inside of left mainshaft case bearing 42) to the outside of bearing shoulder 32 (i.e. the inside of right mainshaft trapdoor bearing 46) is 5.97 inches. This distance is characteristic of Big Twin five speed transmissions. Any mainshaft that is to be used in one of these transmission cases without need to modify the transmission case or the trapdoor, must have bearing surfaces that register with left mainshaft case bearing 42 and right mainshaft trapdoor bearing 46 that are separated by 5.97 inches. The same is true of any countershaft that is to be used in one of these transmission cases without need to modify the transmission case or the trapdoor, in that such a countershaft must have bearing surfaces that register with a left countershaft case bearing 44 and a right countershaft trapdoor bearing 48 that are separated by 6.10 inches (as measured from the inside of left countershaft case bearing 44 to the inside of right countershaft trapdoor bearing 48).
Referring now again to FIG. 1A, when assembled, five speed gear set 60 is located inside housing 12, with trapdoor 18 fastened to the right side of housing 12, with shifting assembly 94 fastened to the top of hosing 12, and the appropriate covers (for example, transmission top cover and transmission end cover) and other components in place.
There are few modern motorcycles that have transmission cases separate from the engine case. BMW motorcycles, for example, have transmission cases that are bolted to the engine case, but that can be unbolted during service. Historically, more motorcycles had separate engine and transmission cases. The modern trend, for motorcycles other than Big Twins, seems to be toward unitary engine and transmission cases.
Motorcycle transmission cases, in general, are designed to be as small as possible because of weight and size considerations. This is especially true when the transmission case is separate and therefore already heavier than if it were combined with the engine case. Thus, the cavity inside the transmission case is generally small.
Another reason why the transmissions for Harley-Davidson.RTM. Big Twin motorcycles is unique is that the large amount of torque generated by the engines available on those motorcycles has, historically, lead to little attention being given to optimizing the gearing. Harley-Davidson made some improvements, for example, by introducing 4 speed transmissions in the 1930's and then introducing 5 speed transmissions in the 1980's. Adding additional gears allows one to take better advantage of narrower power bands. However, because Harley-Davidson.RTM. Big Twin motorcycles have so much torque and such a wide power band, adding additional gears has not been a high priority. In racing motorcycles, adding additional gears to take better advantage of the narrower power bands on racing-tuned engines is common, but that is not the case on Big Twins.
On Big Twins, the more prevalent concern is to lower the engine r.p.m. at cruising speed. Often, this is done by replacing the stock rear pulley with a smaller one. This has the effect of increasing the motorcycle speed for any given engine r.p.m., or conversely, to reduce the engine r.p.m. for any given speed. Although reducing the size of the rear pulley lowers cruising r.p.m. (or, conversely, increases cruising speed), it effects performance at all the gears.
Custom, replacement gear sets are also available that modify some of the stock gear ratios. For example, the assignee of this patent, Custom Chrome, offers its RevTech.RTM. gear sets for 5 speed Big Twin transmissions in either the stock 3.24:1 or close-ratio 2.94:1 first gears. However, because of size limitations of the transmission case cavity, the changes that have been possible through replacement gear sets have been minor.
Complete custom transmissions (often larger than stock) are also available if more significant modifications are required than are permitted by the size limitations of the transmission case cavity. However, replacing a transmission case can take in excess of 8 hours if performed by a trained mechanic. If a less-trained, weekend motorcycle-enthusiast would undertake such a task, it could probably mean having a dismantled motorcycle in the garage or shop for several days.
Thus, the need exists for a method or apparatus for providing low cruising r.p.m. for Big Twin motorcycles without affecting the other transmission characteristics.
The need also exists for increasing top-end speed of Big Twin motorcycles without affecting the other transmission characteristics.
The need also exists for gear configurations that allow a more compact arrangement of gears inside a Big Twin transmission.
The need also exists for a gear set that is stronger and with a longer life.
The need also exists for Big Twin transmissions in which the shift pawl can be adjusted when the gears are in more than one configuration (i.e. other than in third gear).
The need also exists for Big Twin transmissions that provide smoother, more uniform shifting, with the same lever throw in every gear.
The need also exists for a method and apparatus for fulfilling the above needs that is not detectable from a visual inspection of the assembled motorcycle.
The need also exists for a method and apparatus for fulfilling the above needs with a minimum amount of work and expense on the part of the motorcycle owner or mechanic.